The discovery that many organic compounds in organic solvents are capable of stimulated emission by means of optical excitation from a pumping laser has led, in the past ten years, to widespread use of dye lasers as a tunable monochromatic light source. The major advantage of dye lasers, as compared to gas or solid lasers, is that narrow band and continuously tunable emission wavelengths can be selected, by use of a dispersive element for various applications out of the large fluorescent spectral range of organic dyes.
Dye lasers typically contain a solution of an organic laser dye dissolved in an organic solvent such as an alcohol, dioxane or N,N-diethylformamide. The laser dye solution is pumped in a closed circuit through a dye flow cell (flow through laser dye cell, where it is optically excited to laser emission by means of a pumping laser. The laser dye solution is usually cooled to dissipate heat generated by the pumping laser during optical excitation.
Today about 100 laser dyes are used to cover the spectral range from 350 to 1200 nm.
The use of laser dyes in organic solvents as laser-active mediums in conventional manner has severe drawbacks.
The optical excitation by means of the pumping laser liberates heat in the solvent. The refractive index of an organic solvent depends strongly on its temperature (about -4.times.10.sup.-3 [K.sup.-1 ]), and as a result the liberated heat causes striation ("Schlieren") and flow fluctuations in the laser-active dye medium. The heat induced fluctuations and striations in the laser-active dye medium in turn degrade and lower the laser beam quality of the dye laser.
Other drawbacks of the organic solvent include flammability, risk of explosion, and toxicity. Water would be an ideal solvent for dye lasers because it is safe to handle, has a high heat capacity and a very low temperature dependence of its refractive index (-0.09.times.10.sup.-3 [K.sup.-1 ]).
However, few laser dyes are adequately soluble in water. Such laser dyes have a penchant toward dimerization, resulting in an adverse effect on dye laser emission. Dimerization is a function of laser dye concentration. Dimerization begins to occur at laser dye concentrations of 10.sup.-4 mol/l, which is within the laser dye concentration range used in dye lasers.
To date, poor solubility of most laser dyes in water has ruled out the possibility of the use of water as a solvent in dye lasers.
A laser-active aqueous medium is described by K. L. Matheson, J. M. Thorne, Appl. Phys. Lett. Volume 33, (9) (1978), pages 803 to 804, in which the laser dye is dissolved in organic solvents and this solution is subsequently emulsified in water. This process requires that the organic solvent exhibit the same refractive index as water. However, due to the inherent time-related instability of aqueous emulsions of organic solvents, the dimerization of high dye concentrations and the constraints on dye/solvent composition with respect to its emulsibility in water, this laser-active aqueous medium is only usable for special applications.